Weather resistant granules of various hues, embedded on the surface of various types of roofing and/or siding, have been used extensively to provide an aesthetically pleasing color to such surfaces. In North America, asphalt-based roofing materials are a popular medium for covering roofs of homes and other structures. Asphalt-based roofing materials typically come in shingle or roll form, the shingle being the more widely used material. A typical asphalt shingle has an asphalt substrate and a multitude of granules placed thereon. The granules are generally embedded in the asphalt coating on the surface of an asphalt-impregnated base material such as roofing felt. The granules, which are embedded in one surface of the asphalt-impregnated and/or asphalt-coated fiber sheet material, form a coating to provide an inherently weather-resistant, fire-resistant, and/or decorative exterior surface. Because the granule coating provides the aesthetic effect observable with respect to the roofing material, the appearance of the granules is of major marketing importance. For this reason, a pigmented color coating is ordinarily applied to the granules to enhance their visual decorative effect.
The granules employed for roofing and siding are generally derived from a hard mineral base rock such as slate, basalt or nephelite. The granules are typically ground to a particle size of about 10 to about 35 US mesh. These granules, which comprise a base substrate, are then coated with a pigment composition having a thickness of about 4 to about 18 microns. Colored roofing granules are typically prepared by heating mineral rock granules of about 12-40 US mesh up to approximately 1000° F. A paint slurry containing a pigment is then applied to the heated granules in a mixer. Kaolin clay is used extensively in silicate paint formulations for coloring roofing granules. It serves as a filler, extender, moisture release agent, and reactant to aid film insolubilization during high temperature firing. Although kaolin clay is a major component of such coating formulations, it alone lacks the brightness and opacity needed to hide the dark underlying base rock of the granule. Light colored roofing granule coatings using natural kaolin clay frequently require appreciable amounts of TiO2 to achieve desired color specifications. Light colored roofs are particularly favored in many areas. The TiO2 is commonly used in conventional insolubilized alkali silicate coatings, such as those described in U.S. Pat. Nos. 2,379,358; 3,255,031; and 3,479,201, which are incorporated herein by reference. Other types of coating compositions have been used. For example, U.S. Pat. No. 3,528,842, which is incorporated herein by reference, discloses artificially colored roofing granules. These granules consist primarily of crushed and screened minerals, in which the granules are coated with a suitable pigment contained in an inorganic matrix or bond. Some common pigments include red iron oxide, yellow iron oxide, titanium dioxide, chrome hydrate, chrome oxide, chrome green, ultramarine blue, phthalocyanine blue and green, carbon black, metal ferrites, and mixtures thereof. The bond is usually applied in the form of a soluble silicate solution and is insolubilized either by heat treatment or by a combination of heat treatment and chemical action. Minnesota Mining and Manufacturing Company (“3M”) has offered pigmented roofing granules known under the trade designations “LR-7000,” “LR-7070,” 3M Brand “4100,” and 3M Brand “9300.” These granules include multiple silicate-clay coatings which may include cuprous oxide or zinc oxide. Other types of pigment compositions include an aqueous slurry of an iron oxide pigment and a mixture of a silicate and clay.
Although the color of a particular roofing system plays an important role in determining which roofing system will be purchased by consumers, other factors are also of importance in determining the color of a particular roofing system. One such factor is the energy efficiency of the roofing system. Several cities and states are beginning to consider legislation or currently require some roofing structures to have a certain resulting reflectivity. This is of particular importance in temperate regions, such as in the Southern and Southwest regions of the United States. For instance, California is planning to require in 2005 “cool roof” systems to have a resulting reflectively of at least 70% and an emmissivity of at least 0.9. Lighter colored roofs are known to reduce the roof temperature, thereby reducing the cooling costs of the roofed structure. For instance, on a 90° F. sunny day, the roof temperature of a light colored granular coated roof can be about 20-30° F. cooler than an aluminum roof system and about 70-80° F. cooler than a black asphalt roof. As a result, the lighter colored roof system can amount to an energy savings of about 5-10%. The cooler temperature of the light colored roof system not only reduces the cooling costs associated with the building, but the heat generated by the light colored roof contributes less heat to the surrounding environment, thus improving air quality, especially in urban areas.
Typical light colored roofing systems have light colored granules having a reflectivity of up to 30% and a resulting roofing reflectivity of up to 27%. Darker colored granules typically have a lower reflectivity and a resulting roofing reflectivity that is less than roofing systems having lighter colored granules. The Garland Company has recently developed a highly reflective granular roof system that has a resulting reflectivity of at least 45%. This granular roofing system is disclosed in United States Patent Publication No. 20040071938 published on Apr. 15, 2004. Although this granular roof system constituted a significant advance in the roofing, the achievable resulting reflectively on granular roofing systems form on a bitumen layer was limited to less than 70%. The bitumen layer on the roofing system is a generally black color, thus has a low reflectivity. When coating the bitumen layer with reflective particles, some of the bitumen surface remains exposed, thus reducing the resulting reflectivity of the roofing system.
One practice for obtaining a highly reflective surface on a roofing system is to apply a thick coating of highly reflective white paint to the surface of the installed roofing system. Typically a wet paint coating thickness of about 32 mils or more was applied after the roofing system is installed on a structure. Although the application of a highly reflective paint layer on the existing roof system achieved a resulting reflectivity of 70% or greater, the application of the paint on the roof system was expensive due to the large volume of paint needed to obtain the needed paint coating thickness to achieve the desired resulting reflectivity, and from the labor and materials required to apply the paint to the surface of the roofing system. Furthermore, the application of a paint coating on an existing roof system can be problematic due to the problems associated with paint spray during application of the paint and/or the added weight to the roof system due to the application of the thick paint layer.
In view of the continued demand for greater energy savings, there is a demand for a prefabricated roofing system having increased reflectivity.